CLUSTER

Name

Synopsis

Description

CLUSTER instructs PostgreSQL to cluster the table specified by
tablename based on the index
specified by indexname. The index
must already have been defined on tablename.

When a table is clustered, it is physically reordered based on
the index information. Clustering is a one-time operation: when
the table is subsequently updated, the changes are not clustered.
That is, no attempt is made to store new or updated rows
according to their index order. If one wishes, one can
periodically recluster by issuing the command again.

When a table is clustered, PostgreSQL remembers on which index it was
clustered. The form CLUSTER tablename reclusters the table on the
same index that it was clustered before.

CLUSTER without any parameter
reclusters all the tables in the current database that the
calling user owns, or all tables if called by a superuser.
(Never-clustered tables are not included.) This form of
CLUSTER cannot be called from inside a
transaction or function.

When a table is being clustered, an ACCESS
EXCLUSIVE lock is acquired on it. This prevents any other
database operations (both reads and writes) from operating on the
table until the CLUSTER is finished.

Parameters

indexname

The name of an index.

tablename

The name (possibly schema-qualified) of a table.

Notes

CLUSTER loses all visibility
information of tuples, which makes the table look empty to any
snapshot that was taken before the CLUSTER command finished. That makes CLUSTER unsuitable for applications where
transactions that access the table being clustered are run
concurrently with CLUSTER. This is most
visible with serializable transactions, because they take only
one snapshot at the beginning of the transaction, but
read-committed transactions are also affected.

In cases where you are accessing single rows randomly within a
table, the actual order of the data in the table is unimportant.
However, if you tend to access some data more than others, and
there is an index that groups them together, you will benefit
from using CLUSTER. If you are
requesting a range of indexed values from a table, or a single
indexed value that has multiple rows that match, CLUSTER will help because once the index
identifies the heap page for the first row that matches, all
other rows that match are probably already on the same heap page,
and so you save disk accesses and speed up the query.

During the cluster operation, a temporary copy of the table is
created that contains the table data in the index order.
Temporary copies of each index on the table are created as well.
Therefore, you need free space on disk at least equal to the sum
of the table size and the index sizes.

Because CLUSTER remembers the
clustering information, one can cluster the tables one wants
clustered manually the first time, and setup a timed event
similar to VACUUM so that the tables are
periodically reclustered.

Because the planner records statistics about the ordering of
tables, it is advisable to run ANALYZE on the newly clustered table.
Otherwise, the planner may make poor choices of query plans.

There is another way to cluster data. The CLUSTER command reorders the original table using
the ordering of the index you specify. This can be slow on large
tables because the rows are fetched from the heap in index order,
and if the heap table is unordered, the entries are on random
pages, so there is one disk page retrieved for every row moved.
(PostgreSQL has a cache, but the
majority of a big table will not fit in the cache.) The other way
to cluster a table is to use

CREATE TABLE newtable AS
SELECT columnlist FROM table ORDER BY columnlist;

which uses the PostgreSQL
sorting code in the ORDER BY clause to
create the desired order; this is usually much faster than an
index scan for unordered data. You then drop the old table, use
ALTER TABLE ... RENAME to rename
newtable to the old name, and
recreate the table's indexes. However, this approach does not
preserve OIDs, constraints, foreign key relationships, granted
privileges, and other ancillary properties of the table — all
such items must be manually recreated.

Examples

Cluster the table employees on the
basis of its index emp_ind:

CLUSTER emp_ind ON emp;

Cluster the employees table using the
same index that was used before:

CLUSTER emp;

Cluster all tables in the database that have previously been
clustered: